Coated heat spreaders

ABSTRACT

A coated heat spreader for a die includes a body and a coating on a surface of the body, wherein the outermost coating is an organic surface protectant. An IC package includes a die thermally coupled to a heat spreader coated with an organic surface protectant. A PCB assembly including a die thermally coupled to a heat spreader coated with an organic surface protectant, where the die is part of an IC package or is directly attached to the PCB. A method of making a coated heat spreader includes coating the organic surface protectant onto a surface of the heat spreader.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to thermal heat spreaders, integratedcircuit (“IC”) assemblies containing heat spreaders, and methods ofmaking the heat spreaders and assemblies.

[0003] 2. Related Art

[0004] An IC die may be mounted on a substrate to form an IC assembly.For example, a die may be mounted on a package substrate to form an ICpackage, or the die may be mounted directly to a printed circuit board(“PCB”). To dissipate heat from the die, a heat spreader is typicallythermally coupled to the back of the die. Generally, there is a thermalinterface material (“TIM”) between the die and the heat spreader. Pooradhesion between the TIM and the heat spreader is a common problem.

[0005] There is therefore a need to improve adhesion between the heatspreader and the TIM. There also is a need to improve such adhesion in acost-effective manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Features and details of the invention can be found in theillustrative embodiments of the invention which are described below withreference to the drawings, in which:

[0007]FIG. 1 shows a cross-section of an embodiment of a coated heatspreader,

[0008]FIG. 2 shows an embodiment of an IC package that includes asemiconductor die and a coated heat spreader, and

[0009]FIG. 3 shows a process flow diagram for manufacturing anembodiment of a coated heat spreader.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010]FIG. 1 shows an embodiment of a heat spreader 10 of the invention.Heat spreader 10 includes body 30, which is made of copper. The heatspreader body 30 may be made of any thermally conductive metal or alloy.Preferably, the body is composed of copper, nickel, aluminum, tin, orgold metal, or alloys thereof. More preferably, the body is composed ofcopper.

[0011] The body may have a variety of shapes depending on the specificapplication, but generally the body provides a surface for efficientthermal coupling to the die. A preferred shape for the body is asubstantially flat surface, for abutting the die, and sidewalls thatflank the die and descend down from the flat surface to the packagesubstrate, such that the body has the general shape of a lid or cap. Inother embodiments where the die is directly mounted on a PCB, thesidewalls may descend down from the flat surface to the PCB. Heatspreader body 30 is an example of such a lid shape. The body may extendsignificantly beyond the periphery of the die as needed to dissipateheat by an appropriate amount for the particular die or package.

[0012] An organic surface protectant (“OSP”) coating 20 lines thesurface of the heat spreader body 30. In the embodiment of FIG. 1, theOSP coating 20 is formed by applying COBRATEC® 939, which is a blend ofazoles. Any suitable OSP may be used, e.g., any of the organicsolderability preservatives used for coating PCB lands. Organicsolderability preservatives are used to coat PCB lands that are sitesfor solderjoints. This prevents oxides, which can jeopardize goodsolderjoint adhesion, from forming on the surface. Examples of suppliersof organic solderability preservatives include Enthone, Inc. (WestHaven, Conn.), Kester Solder (Des Plaines, 111.), PMC Specialties Group,Inc. (Westlake, Ohio), and Tamura Kaken Co. Ltd. (Tokyo, Japan).

[0013] The OSP preferably comprises one or more substituted orunsubstituted imidazole or triazole compounds in aqueous solution.Examples of imidazole compounds include benzimidazoles. Examples oftriazole compounds include but are not limited to benzotriazole,tolyltriazole, carboxybenzotriazole, and sodium tolyltriazole, andpotassium or sodium salts thereof. OSPs may also be based on organicesters of dicarboxylic acids. Examples of OSP products include theCOBRATEC® (PMC Specialties Group, Inc.) line of metal protectionproducts, e.g., COBRATEC® 99, COBRATEC® 45-I, COBRATEC® CBT, COBRATEC®939, and COBRATEC® 948. OSPs from other manufacturers include, e.g.,Entek Plus 56 and 106A (Enthone, Inc.), and Protecto® 5630 and 5631(Kester Solder).

[0014] The OSP may be provided as a liquid or as a solid that is mixedprior to coating a heat spreader. For example, an OSP liquid may contain0.5-2.0 wt % of solids. The OSP may be blended with a co-solvent.Preferred co-solvents are polar solvents. Particularly preferred polarco-solvents are glycols, alcohols, and aminoalcohols. Examples ofco-solvents include but are not limited to ethylene glycol, diethyleneglycol, propylene glycol, isopropanol, and triethanolamine. The OSPsolution may be heated during step 90, e.g., to a temperature of from35° C. to 50° C.

[0015] The thickness of the OSP coating on the coated heat spreader mayvary, e.g., depending on the particular OSP and the deposit time. Forexample, for certain OSPs the coating thickness may range from 0.1 μm to1.0 μm or from 0.2 μm to 0.5 μm.

[0016] In the embodiment shown in FIG. 1, the entire surface of body 30is coated. In other embodiments, however, the surface of the heatspreader may be partially coated.

[0017] Once applied to a heat spreader, the presence of an OSP coatingmay be verified on a test coupon using gas chromatography, UV(ultraviolet) spectroscopy, or mass spectrometry. The OSP coating mayimpart a visible coloration to the heat spreader body, e.g., a triazoleimparts an orange-blue tinge to a copper-based heat spreader.

[0018] In one embodiment of the invention, an IC die that is thermallycoupled to a coated heat spreader is incorporated into a PCB assembly aspart of an IC package. Alternatively, the IC die may be directlyattached to the PCB.

[0019]FIG. 2 shows an embodiment of an IC package containing coated heatspreader 10. Flip chip die 15 is mounted on package substrate 40 via diebumps and underfill 18. Alternatively, die 15 may be configured formounting by other means besides a ball grid array (“BGA”). For example,flip chip die 15 may be mounted by a pin grid array (“PGA”). The flipchip die may be any active or passive electronic device, e.g., amicroprocessor or a memory chip. Package substrate 40 includes pads 65and solder bumps 70 for mounting to a PCB. In other embodiments,substrate 40 may be mounted to a PCB by other means.

[0020] Heat spreader 10 takes the form of a cap, with sidewallsextending down to package substrate 40. A surface of heat spreader 10 isadjacent and thermally coupled to TIM 45.

[0021] The TIM 45 in FIG. 2 is a solder. In other embodiments, TIM 45may be of a variety of materials, such as organic, inorganic, or hybridmaterials. Inorganic TIMs may include any solder material, e.g.,conventional solders such as alloys of zinc and copper, and alloys oftin, e.g., eutectic tin/lead, tin/silver/copper, or tin bismuth. Inprinciple, any metal or metal alloy solder may be used as a solder TIM.Examples of other alloys include Sn/Pb/Ag, Sn/Ag/Cu/Sb, Sn/Zn/Bi, andSn/Zn. Suitable alloys may be readily obtained from commercialsuppliers, e.g., Multicore, AIM, and SDK. Organic TIMs generally adherewell to heat spreaders that are not coated, because oxide formation istolerable. Organic TIMs can maintain good adhesion despite surfaceoxides on the heat spreader. Organic TIMs may be, e.g., made of polymer.Heat spreaders with surface oxides may adhere poorly to inorganic andinorganic-organic hybrid TIMs, since oxides generally jeopardizeinorganic joint adhesion. Inorganic-organic hybrid TIMs may be, e.g.,solder-polymer TIMs. For these TIMs, the heat spreader must generally becoated.

[0022] One solution is to plate the heat spreader with a gold layer overa nickel layer (“Au/Ni finish”) to protect against corrosion bypreventing oxides from forming. Applying an Au/Ni finish, however,involves a costly plating process with long through put times, andrequires disposing of environmentally unfriendly plating bath chemicals.And the Au/Ni finish provides poor adhesion for organic material inmoisture and thermal cycling conditions common in industrial electronicsprocesses. Because of the moisture levels and thermal cycling common toelectronics industry processes, TIMs made of inorganic-organic hybridsmay not adhere well to heat spreaders coated with the Au/Ni finish. Thusthe Au/Ni finish may provide an unsatisfactory solution to the problemof oxide formation for inorganic-organic hybrids TIMs.

[0023] An OSP coating on a heat spreader can provide an improved wettingsurface for both TIMs made of inorganics and TIMs made ofinorganic-organic hybrids. Compared to Au/Ni plating processes, OSPcoating processes typically require lower through put time, less floorspace, and less costly equipment.

[0024] The IC package of FIG. 2 further includes a heat sink 50, whichis thermally coupled to heat spreader 10 via a second TIM 60. The secondTIM may be selected as needed from any materials suitable for use with aheat sink.

[0025]FIG. 3 shows a flow diagram illustrating one embodiment of amethod for coating a heat spreader body with an organic surfaceprotectant (“OSP”) in accordance with the present inventions. In step80, the heat spreader body 30 is chemically cleaned to remove oxidationand oil residue that may be present on the surface of the heat spreaderbody. Preferably, chemical cleaning step 80 is carried out in alkalinesolution. The cleaning solution may be heated above room temperatureduring chemical cleaning in step 80.

[0026] After chemical cleaning, the heat spreader body is microetched inacid solution in step 82 to provide a matte texture to the surface ofthe body. Microetching may be carried out in any suitable acid solution,e.g., aqueous nitric acid. The acid solution may be heated slightlyabove room temperature during etching step 82.

[0027] After microetching, the heat spreader body undergoes a waterrinse in step 84, an acid rinse in step 86, e.g., using 5-10% sulfuricacid, and then another water rinse in step 88.

[0028] The heat spreader body is now ready for OSP application in step90. Step 90 is carried out by dipping the heat spreader body 30 in asolution of the OSP. The dipping dwell time in step 90 may be varied asneeded to provide an effective coating. For dipping a plurality of heatspreader bodies, the bodies are preferably placed into a strainer insuch a way as to minimize contact. Alternatively, the OSP may be appliedby spraying the heat spreader body with a solution of the OSP.

[0029] An OSP coating may be directly applied to the heat spreader bodysurface as in the preferred embodiment shown in FIG. 3. Alternatively,the heat spreader body surface may be first coated with another materialand then coated with the OSP. For example, a copper heat spreader bodymay be plated first with nickel or palladium, then coated with the OSP.

[0030] After the application of the OSP, the coated heat spreader body30 is then rinsed in de-ionized water at step 92. The coated heatspreader is then dried in step 94, after which it is ready forinspection, followed by packaging, shipping, and storage. Preferably,the coated heat spreader should be packaged and stored in a low humidityenvironment in a manner that avoids physical contact to deviceattachment sites. Preferably, the coated heat spreaders are handled onlyat the edges thereof or using gloves, because human hands may transferoils containing corrosive salts and acids, which are a detriment toadhesion.

[0031] It should be noted that the specific chemistries, concentrations,and temperatures for the various baths used in the steps of FIG. 3 willvary depending on the particular OSP and heat spreader body used. Forexample, an OSP supplier may provide recommendations for applying theparticular OSP of that supplier.

[0032] While embodiments of the invention have been described above,those embodiments illustrate but do not limit the invention. Adaptationsand variations of those embodiments are within the scope of theinvention as set forth in the following claims.

We claim:
 1. A method comprising: providing a thermally conductive heatspreader body having a first surface configured to thermally couple theheat spreader to an IC die; and coating the first surface with anorganic surface protectant.
 2. A method as claimed in claim 1, whereinthe coating step comprises immersing the heat spreader body in a dippingsolution comprising the organic surface protectant.
 3. A method asclaimed in claim 1, wherein the organic surface protectant comprises oneor more triazole compounds and/or salts thereof.
 4. A method as claimedin claim 1, further comprising providing the coated first surface with alayer of a thermal interface material, and thermally coupling the IC dieto the heat spreader body via the thermal interface material.
 5. Amethod as claimed in claim 4, wherein the thermal interface material isa solder or solder-polymer hybrid.
 6. A method as claimed in claim 1,wherein the first surface of the heat spreader body is coated with anintervening layer before coating with an organic surface protectant. 7.A method as claimed in claim 1, wherein the body comprises copper.
 8. Aheat spreader for an IC circuit package, comprising: a thermallyconductive heat spreader body having a first surface configured tothermally couple the heat spreader to an IC die; and a coating oforganic surface protectant on the first surface.
 9. A heat spreader asclaimed in claim 8, wherein the body comprises copper.
 10. A heatspreader as claimed in claim 8, wherein the organic surface protectantis applied by dipping or spraying onto the first surface.
 11. A heatspreader as claimed in claim 8, wherein the organic surface protectantcomprises one or more triazole compounds or salts thereof.
 12. A heatspreader as claimed in claim 8, wherein the coating completely envelopsthe body.
 13. An IC package, comprising: a package substrate; an IC dieattached to the substrate; a heat spreader body having a first surfacethermally coupled to the IC die; and a coating of organic surfaceprotectant disposed between the first surface and the IC die.
 14. An ICpackage as claimed in claim 13, wherein the coating completely envelopsthe body.
 15. An IC package according to claim 13, further comprising athermal interface material between the IC die and the coated firstsurface of the heat spreader.
 16. An IC package according to claim 15,wherein the thermal interface material is a solder or solder-polymerhybrid.
 17. An IC package according to claim 13, wherein the bodycomprises copper.
 18. An IC package according to claim 13, wherein theorganic surface protectant comprises one or more triazole compounds orsalts thereof.
 19. An IC package according to claim 13, wherein theorganic surface protectant is in indirect contact with the firstsurface.
 20. A printed circuit board assembly comprising: a printedcircuit board, an IC die electronically coupled to the printed circuitboard, and a heat spreader body having a first surface thermally coupledto the IC die; and a coating of organic surface protectant disposedbetween the first surface and the IC die.
 21. A printed circuit boardassembly as claimed in claim 20, further comprising a thermal interfacematerial between the IC die and the coated first surface of the heatspreader, wherein the thermal interface material is a solder orsolder-polymer hybrid.
 22. A printed circuit board assembly as claimedin claim 20, wherein the IC die is directly attached to the printedcircuit board.
 23. A printed circuit board assembly as claimed in claim20, wherein the IC die is attached to a package substrate that isattached to the printed circuit board.